Feasibility study on multi-seam upward mining of multi-layer soft–hard alternate complex roof

Multi-seam upward mining is of considerable importance to improve the resource recovery rate and realize waste-free mining. Taking the Dianping coal mine in Shanxi Province as an example, the development height and evolution process of the overburden fracture after mining of the Taiyuan Group coal seam were studied by combining theoretical analysis, field measurement, and numerical simulation to verify the feasibility of upward mining (UM) of the multi-layer soft–hard alternate complex roof of Taiyuan group coal seam in Shanxi Province, China. A new method for determining the development height of overburden fractures (DHOF) is proposed on the basis of analyzing the fracture characteristics of rock strata with different lithologies. This method first calculates the fracture of key strata and then that of key soft rock strata controlled by key strata. The above method found that the DHOF of the Dianping coal mine is 52.5 m, and the field measurement and numerical simulation results are 53 and 49.7 m, respectively. The numerical simulation study found that after the upper and lower coal seams were mined, the rock fissures between the two seams were only connected to a small extent near the open-off cut, and most of the area still impermeable. Therefore, using the proposed method in UM is feasible after simple reinforcement of the rock seam in the vicinity of the open-off cut. In addition, on-site field measurements applied the borehole resistivity method to present a dynamic all-round view of the overburden fracture development process, and monitoring results indicate the existence of an unconnected fracture zone at the top of the fracture zones. Research results provide an important theoretical and technical basis for the prediction of the development height of the overlying rock fracture zone and the feasibility of UM in the Taiyuan coalfield, Shanxi Province.